This invention relates to a method and a system for treatment of water and wastewater. More specifically, the method and system of the present invention are designed for the biological removal from wastewater of contamination in the form of solids and soluble organic material, and optionally the biological removal of nitrogen and phosphorus nutrients.
The biological treatment of sewage and other apparatuses for effecting such treatment are described in U.S. Pat. Nos. 2,907,463, issued to D. J. N. Light et al,. on Oct. 6, 1959; 3,964,998, issued to J. L. Bernard on Jun. 22, 1976; 4,279,753 issued to N. E. Nielson et al. on Jul. 21, 1981; 4,430,224 issued to U. Fuchs on Feb. 7, 1984; 4,468,327 issued to A. D. Brown on Aug. 28, 1984; 4,522,722 issued to E. M. Nicholas on Jun. 11, 1985; 4,663,044, issued to M. C. Goronszy on May 5, 1987; 4,798,673 issued to C. Huntington on Jan. 17, 1989; 4,867,883 issued to G. T. Daigger on Sep. 19, 1989; 4,948,510 issued to M. D. Todd et al. on Aug. 14, 1990; 5,013,441 issued to M. C. Goronszy on May 7, 1991; and in Canadian Patents Nos. 997,488, issued to B. K. Tholander et al. on Sep. 21, 1976 and 1,117,042, issued to M. L. Spector on Jan. 26, 1982.
In general, suspended growth activated sludge processes and modifications thereto have recently been shown to be able to accomplish the objectives set out above, but the costs of specialized clarifiers required to settle and recycle biological solids are quite high. Moreover, the size, complexity and operating problems encountered with such systems make them unattractive to many potential users. In addition, the basic activated sludge process often results in poor treatment because of the continuous flow through characteristics in the final clarifier and the resulting constant sludge management requirements.
As a consequence of the foregoing, during the past ten years improved final clarification and solids removal has been sought by using larger and improved but more complex and expensive designs for final clarifiers. Moreover, final effluent filtration is often necessary to reliably meet permitted environmental standards.
There has also been a recent revival of interest in the sequencing batch reactor (SBR) suspended growth activated sludge process because of the inherently more efficient batch settling and often higher treatment efficiency for batch organic contaminants removal than is possible with the conventional continuous flow activated sludge process. The SBR process uses the same vessel for batch aerated biological reaction and quiescent batch settling. Thus, the SBR process eliminates the major cost of dedicated final clarifiers necessary for the conventional activated sludge process as well as improving solids removal performance.
However the SBR process has several disadvantages, the principle one being that it does not operate with a constant level and continuous flow, but requires intermittent operation for cycles of fill, react, settle, decant and idle. Typical level fluctuations are 30% to 50% of the maximum operating depth or as much as 5 to 10 feet of level fluctuations. The result is a much lower use to total volume ratio than the conventional activated sludge process. Accordingly, the SBR process is generally not cost effective for flows greater than five to ten million gallons per day (MGD).
Another disadvantage of the SBR process is that significant head loss occurs from the influent to the final effluent, requiring additional energy and pumping costs. Additionally, because the effluent flow is not continuous, flow equalization systems may be required to prevent peak loadings and adverse impacts on receiving waters.
Finally, the basic process and design limitations of the SBR process make it difficult to achieve the same high efficiency biological nutrient removal possible using the continuous flow activated sludge process, especially for weak or colder wastewaters.
Several improvements have been attempted to overcome the limitations of the conventional SBR process. A continuous inflow, partitioned SBR process U.S. Pat. No. 4,468,327 was issued to A. D. Brown on Aug. 28, 1984, and cyclically operated intermittent flow path sequencial cycle, multi-zoned recycle SBR process U.S. Pat. Nos. 4,663,044 and 5,013,441 were issued to H. G. Goronszy on May 5, 1987 and May 7, 1991 respectively. Significant level fluctuations, head losses and intermittent high flow rate discharges, however, still prevent these processes from overcoming all the limitations of the conventional SBR process.
Attempts have also been made over the years to overcome the level variation limitations of all SBR type processes and the cost of dedicated final clarifiers for the conventional activated sludge process. The Degremont S.A. U.S. Pat. No. 3,470,092 issued to J. J. P. Bernard on Sep. 30, 1969 illustrates a first attempt to develop a new suspended growth activated sludge process utilizing the concepts of both batch treatment and continuous flow. This two cell process was partially interconnected at the water surface. The alternate cell feed concept was not effective because it did not achieve a high treatment efficiency, had a low aerator utilization factor, and required long detention times to operate, resulting in expensive systems. Union Carbide U.S. Pat. No. 4,179,366 issued to J. R. Kaelin on Dec. 18, 1979 added a third bottom interconnected cell, but also suffered from low treatment efficiency and ineffective changeover of untreated wastewater from the first cell to the third cell. The processes disclosed by both patents also required significant level fluctuations in the treatment cells between operating cycles which made it difficult to control flows and operate fixed, level-sensitive mechanical aeration systems.
Linde AG German Patent No. 3,147,920 issued on Oct. 13, 1983 utilized the same three cell concept as U.S. Pat. No. 4,179,366. Although this three cell process achieved a more constant level, and overcame some of the limitations of the prior art, the process failed because it relied on expensive and unreliable mechanical gates to separate the treatment cells at various cycle times, and because treatment efficiency and effectiveness was too low to be commercially useful.
VOR SA French Patent No. 2550522 issued on Feb. 15, 1985 describes another constant level apparatus including three separate, identical basins. This process required a large, expensive treatment system because three independent basins were required, only 1/3 of the total treatment volume was used for biological treatment at any time, and only 1/3 of the aeration equipment could be used at one time.
In spite of these attempts to improve on the performance and effectiveness of the SBR and conventional activated sludge processes, they do not provide a higher treatment efficiency and hence they are not significantly more cost effective. Such attempts have either failed to totally achieve the desired benefits, or have new inherent disadvantages which result in little or no net benefits compared to conventional methods.
All these attempts to develop constant level processes to improve on the conventional suspended growth activated sludge process, rely on the management control and recycle of mixed liquor suspended solids by back flushing or forward flushing through or around the treatment system by control of the timing and direction of wastewater flow into and through the treatment system. These methods of solids management differ significantly from variable level SBR's, and also differ from the constant level conventional activated sludge process, which settles the mixed liquor suspended solids in a dedicated final clarifier to collect and recycle the resulting activated sludge back to the aeration basin.
In contrast, this present invention relies on biological solids management by a novel substantially constant level wastewater treatment method and system in which mixed liquor suspended solids are positively transferred alternatively from at least one last treatment discharge cells to a previous hydraulically connected treatment cell simultaneously with the co-mingled partially treated wastewater, while being subjected to mixing prior to each last cell sequentially functioning once again as a quiescent clarifier for discharge of final treated effluent.
This novel treatment method and system makes it possible to achieve many worthwhile objectives to significantly increase the usefulness and lower the cost of wastewater treatment.